Critical Research Failure / Science Marches On: Even in view of their twisted horrific circumstances, the Westermarck Effect would surely have prevented Chris and Cathy from having any interest in each other. Corrine and Christopher Sr.'s incestuous relationship makes sense only because they were raised apart from each other and therefore could not develop the reversed sexual imprinting that typically prevents such incestuous desires from developing in the first place. While Westermarck's research was not widely known or confirmed at the time of this book's publishing, there was already plenty of historical anecdotal evidence for this natural phenomenon underlying the incest taboo.
One counterargument to this might also be the (arguable) increase in each other's perceived sexual attractiveness brought on by the stress of their situation.
The other being the argument against the Westermarck effect in general, which not everyone agrees to be accurate on sexual attraction. See above on artistic license.
There's also sheer evolutionary instinct due to Cathy and Chris effectively becoming Cory and Carrie's parents. They see each other as the twin's mother and father... subtext to this is that this leads to them seeing each other as mates.
Jerkass Woobie: Olivia the grandmother when her past is revealed in the prequel book Garden of Shadows.